1. Field of the Invention
The invention relates to a switched capacitor charge pump including a capacitor having a first capacitor terminal and a second capacitor terminal, a charging device coupled to the first capacitor terminal, for charging the capacitor, a discharge switch coupled to the first and the second capacitor terminal, for recurrently discharging the capacitor by the closing and opening of the discharge switch in response to a first or second value respectively, of a binary cyclic clock signal from a clock signal source.
The invention likewise relates to a sawtooth oscillator including such a switched capacitor charge pump.
2. Description of the Prior Art
A switched capacitor charge pump of this type is known from Proceedings of the IEEE, Vol. 71, No. 8, August 1983, pp. 941-965, R. Gregorian et al., "Switched-Capacitor Circuit Design", FIG. 1(b). The charging device in this paper is a switch capable of connecting the first capacitor terminal to a first voltage source which carries voltage V1. The second capacitor terminal is connected to a second voltage source which carries voltage V2. In each cycle of the clock signal the capacitor is first discharged by the discharge switch and then charged to a voltage difference V1-V2. In the cycle time interval T a charge dQ equal to C*(V1-V2) where C is the capacitance of the capacitor, is flowing. When the capacitor is being charged, the first voltage source is loaded, so that the voltage V1 and hence the charge dQ changes. The loading of the first voltage source may be reduced by a buffer stage i.e. a first operational amplifier arranged as a voltage follower. The second voltage source is usually the inverting input of a feedback second operational amplifier whose noninverting input is connected to the second voltage V2. The output of the second operational amplifier supplies a signal which is a measure for the charge dQ. The second voltage source carrying voltage V2 could be omitted by selecting this voltage to be equal to ground potential. However, in that case a supply voltage which is negative relative to ground potential will have to be available to the second operational amplifier. The realization of an accurate charge pump i.e. an arrangement supplying on its output a predetermined charge dQ in each cycle interval T thus turns out not to be of a simple kind.